1. Field of the Invention
The present invention relates to a fluid sealing structure and more specifically to a structure for sealing fluid charged in a working chamber formed by two relatively rotatable members.
2. Description of the Prior Art
The fluid sealing structure of this type is adopted, for instance, for a power transmission system for a four-wheel drive vehicle, being called viscous coupling.
FIG. 1(A) is a cross-sectional view for assistance in explaining a viscous coupling by way of example of the fluid sealing structure.
In the drawing, a fluid sealing structure is composed of an inner shaft 1, an outer shaft 3 arranged rotatable and coaxial with respect to the inner shaft 3, and a disk-shaped side wall 5. The outermost circumference of the side wall 5 is welded to an end of the outer shaft 3 and the innermost circumference of the side wall 5 is sealably fitted to the outer circumferential surface of the inner shaft 1 so as to form a fluid working chamber 11.
A viscous fluid such as silicon oil is charged within this fluid working chamber 11. Within this fluid working chamber 11, a plurality of fluid resistance plates 9 fixed to the outer circumference of the inner shaft 1 and a plurality of fluid resistance plates 7 fixed to the inner circumference of the outer shaft 3 are arranged alternately at regular intervals along the axial direction of the shafts 1 and 3.
To rotatably seal viscous fluid within the working chamber 11 formed by the two inner and outer shafts 1 and 3 and the side wall 5, an annular recess 17 is formed in an inner circumferential surface 13 of the side wall 5. Within this annular recess 17, there are provided an annular elastic sealing member 19 made of rubber, for instance and a backup right 21 made of ethylene tetrafluoride resin including copper powder, for instance.
Therefore, when pressure within the working chamber 11 rises, since the sealing member 19 is compressed in the axial direction of the shaft and supported by the backup spring 21, the radially inner portion of the sealing member 19 is brought tight contact with the outer circumferential surface of the inner shaft 1 to prevent the high pressure fluid from leaking from a space between the inner shaft 1 and the side wall 5.
In the prior-art viscous coupling, however, when the coupling is disposed within a gear box of a transfer case or a limit slip differential gear for distributing an engine power to the front and rear wheel shafts in a four-wheel driving vehicle, and therefore dipped within a gear oil there exists a problem in that gear oil enters a space between the sealing member 19 and the backup ring 21
between the outer circumference 15 of the inner shaft 1 and the inner circumference 13 of the side wall 5 as shown by an arrow A in FIG. 1(B), in particular when the pressure within the working chamber 11 is lowered during vehicle stop or when the inner shaft 1 is vibrated in the axial direction thereof to cause an oil pumping action.
Under these conditions, when the plural fluid resistance plates 7 and the plural fluid resistance plates 9 start rotating relative to each other in differential operation to transmit power, the temperature within the working chamber 11 rises and therefore the temperature of the gear oil between the sealing member 19 and the backup ring 21 also rises due to thermal conduction. Here, since the coefficient of expansion of the gear oil is larger than that of the working fluid, the heated gear oil expands at high temperature, so that the sealing member 19 is urged toward the working chamber 11. In other words, since the sealing member 19 is deformed, the lip portion 23 (the inner projection) of the sealing member 19 is urged to enter a space formed between the inner circumferential surface 13 of the side wall 5 and the outer circumferential surface 15 of the inner shaft 1. As a result, the lip portion 23 is damaged due to the differential motion between the two shafts and therefore there exists a problem in that a sealing function will be damaged or the durability of the sealing member is degraded.
To overcome this problem, it is also possible to provide an additional backup ring 21 on the working chamber side. In this method, however, the number of parts increases and further the assembly work of the two backup rings 21 and the sealing member 19 into the recessed portion 17 becomes complicated, thus resulting in a higher cost.